High rate modulator/demodulator for digital signals amplitude modulated on digital data

摘要

A modulator circuit produces an AM modulated output signal by modulating a low rate digital signal on a higher rate data signal. The modulator circuit operates to switch the ground return of a line driver circuit which selectively amplifies the high rate data signal and has its ground return switched in accordance with the lower rate digital signal to produce an output modulated signal of a three level modulation characteristic whereby the modulation index of the signal is selectable by varying the impedance in series with the ground return of the line driver. A demodulator for the output signal of the modulator incorporates circuitry by which the high speed data carrier signal is first retrieved and is then subtracted from the modulated signal to yield a separate high speed data signal and the low speed digital signal with an enhanced signal-to-noise ratio.